Overview
Cascade Heating: Application Range, Sizing, and Optimization
What Is Cascade Heating?
A cascade heating system is a setup in which multiple heat pumps are connected in series or in parallel instead of using a single large unit. This configuration enables the heating output to be adjusted more flexibly and precisely in response to the current heat demand than a single heat pump’s simple on/off control. Manufacturers are increasingly designing heat pumps in a modular way to facilitate their use in cascade systems.
For this reason, planners are increasingly favouring cascade heat pump systems when designing heat pump installations. However, these systems are more complex to plan and control than a single, larger unit.
You can find a summary of this blog and a great introduction to the topic in the video below about sizing, optimization and pratical examples of heat pump cascades.
Heat Pump Cascade: Technical Reasons
Cascade heat pump systems are planned where high output is required, where heating, domestic hot water, and cooling demands fluctuate significantly, or where large systems reach their limits.
The main technical reasons for using cascade heating are:
- Conventional heat pumps operate only in two modes—on or off. By connecting multiple heat pumps, heating output can be better matched to the current heat demand. For small heat loads, a modern, inverter-controlled (modulating) heat pump is often sufficient, as it can adjust output via variable-speed compressors.
- In larger systems, greater fluctuations in demand can no longer be balanced by a single inverter-driven heat pump compressor.
- If different temperature levels are required, each heat pump in the cascade can operate at its optimal performance point.
- For a single large air/water heat pump, the required air volume would be impractically large for various reasons.
Cascade Heat Pump System: Different Objectives
Cascade heating has various objectives and use cases. Heat pumps are connected in parallel to increase total capacity and provide flexible coverage of peak loads. When higher temperatures or different temperature levels are required, the heat pumps are connected in series. In practice, the system design is tailored to specific objectives and boundary conditions.
Parallel Heat Pump Cascade
In a parallel heat pump cascade, all heat pumps operate simultaneously and independently. Each heat pump is directly connected to the supply and return of the heating circuit, sharing the same temperature level and dividing the required total output. The individual heat pumps are activated sequentially as heat demand increases.
Key planning considerations:
- Oversizing the system leads to frequent cycling, causing increased wear and efficiency losses.
- The heat pump concept must be adapted to the division between base load and peak load.
- Different variants should be evaluated to optimize the seasonal performance factor.
- It must be ensured that sufficient energy is available from the source (e.g., ground loops or wastewater heat exchangers) for parallel operation.
- The system concept must be adapted to the available space.
Pros and Cons, Application Cases
The following table summarizes the advantages and disadvantages of parallel cascade heat pump systems:
| Pros | |
| Cons | |
| Application | Multifamily buildings with underfloor heating or existing radiators, hotels with wellness areas, sports facilities, schools, commercial buildings, and office buildings. |
Series Heat Pump Cascade
In a series cascade heat pump system (also called a cascade connection in series), heat pumps are connected one after another, with each stage raising the temperature incrementally.
For example, the first heat pump raises water from 5 °C (41 °F) to 25 °C (77 °F), and the second from 25 °C (77 °F) to 45 °C (113 °F). This is useful where a large temperature lift is needed, such as in industrial applications, older buildings with radiators, or district heating networks with different temperature levels (e.g., underfloor heating throughout the network and higher temperatures for domestic hot water in each building).
| Pros | |
| Cons | |
| Application | Older multifamily buildings with radiators, industrial applications, and local heating networks |
Hybrid Heat Pump Cascade: Combining Series and Parallel Configurations
A combination of series and parallel cascade heat pump systems is used in more complex or larger heating systems with varying requirements for output staging, temperature levels, or redundancy.
Typical Applications are:
- High and Low Temperature Stages
In a series configuration, multiple temperature levels are generated (e.g., 35 °C [95 °F] and 70 °C [158 °F]). The first heat pump raises the temperature to a medium level, the second to a high level. Within a temperature stage, multiple heat pumps can be connected in parallel to scale output. Examples include retrofitting multi-family homes with radiators (50–70 °C [122–158 °F]) and underfloor heating (35–40 °C [95–104 °F]) or providing district heating in two stages. - Process Heat or Domestic Hot Water
Parallel heat pumps are used for load distribution, while series connections distribute the temperature lift over several stages—mainly in larger commercial and industrial plants. - Hybrid Heating Systems with Heat Recovery
Series-connected heat pumps can utilize waste heat (e.g., from wastewater), with a second heat pump raising the temperature to a usable level. At each temperature stage, a parallel cascade covers the load. - Reversible Systems for Heating and Cooling
Reversible heat pump systems designed for both heating and cooling can provide simultaneous heating and cooling by combining multiple heat pump stages with different hydraulic configurations. Typically, two stages are used: the first (one or more parallel heat pumps) covers the base load at moderate temperatures with high efficiency, while the second stage (in series) raises the temperature for high-temperature needs (e.g., domestic hot water).
Such solutions are used in hotels, hospitals, and district heating networks.
Application Examples and Cascade Heat Pump Schematic Diagrams
Selecting the right heat pump type and cascade configuration depends on available heat sources, required output, and specific project requirements. Below are practical examples with corresponding cascade heat pump system diagrams.
Air/Water Cascade Heat Pump System
The major advantage of air/water heat pumps in cascades is that they often require no extensive retrofitting. They are quickly deployable and relatively affordable, making them ideal for cascade heating in urban areas and buildings without full thermal insulation.
Heat pump cascade diagram of a multifamily building with a parallel air-to-water heat pump cascade
In a multifamily building, a parallel air/water cascade heat pump system provides heating and domestic hot water. Three 30 kW units are installed; one covers the base load, while the others are activated as demand increases. Serial buffer tanks separate temperature levels for underfloor heating (low) and domestic hot water (high).
Heat pump cascade diagram of a school with an air-to-water heat pump cascade
In a school, a cascade of four air/water heat pumps flexibly supplies the building and gymnasium. In summer, some heat pumps can be shut down to save operating and maintenance costs.
Heat pump cascade diagram of a commercial production hall with a parallel air-to-water heat pump cascade
In a commercial production hall, most heat is needed as high-temperature process heat. Only a small portion is needed as low-temperature heating. A cascade of two powerful air/water heat pumps is used, with serial buffer tanks maximizing volume in limited space.
For more complex projects with hybrid configurations (e.g., new construction at ProfiCamp Eintracht Frankfurt with three air/water and four brine/water heat pumps, or the district heating network at Wohnquartier Grünheide with three air/water heat pumps), various heat sources are combined in planning and simulation.
Brine/Water Cascade Heat Pump System
Brine/water heat pumps can be connected in parallel to multiple geothermal sources if a single source cannot meet the full heat demand. They can also be connected in series with other heat pumps, such as when providing both underfloor heating and domestic hot water.
Heat pump cascade diagram of a hotel with a serial (booster) heat pump using geothermal energy
In a hotel, two parallel brine/water heat pumps draw energy from a geothermal field and hybrid solar panels, raising the temperature to 40 °C (104 °F). A water/water “booster” heat pump then raises the temperature to 65 °C (149 °F) for high domestic hot water demand.
Water/Water Cascade Heat Pump System
A cascade of one or more water/water heat pumps uses water as both heat source and sink—typically from groundwater, geothermal fields, lakes, or waste heat systems.
Local district heating network serving office, sports, and wellness facilities
A BAFA-funded district heating network with a groundwater heat pump and decentralized cascade heat pump supply, plus a closed power distribution network with photovoltaics.
Cascade Heat Pump System with Wastewater Heat Exchanger
In this setup, wastewater is used as both heat source for waste water heat recovery and as a sink. Depending on wastewater temperature and system efficiency of the , additional heat pumps are connected in series for hot water production.
An example of a cold local heating network in a residential area and an example of a large local heating network with mixed use, including both new and existing buildings as well as a swimming pool, can be seen in the excerpt below from the webinar “Using Wastewater Heat.”
Heat pump cascade diagram of a local heating network with a (booster) heat pump
In a residential district with a cold district heating network, a booster heat pump in series raises domestic hot water to 60 °C (140 °F). The wastewater heat exchanger serves as the source for both heating and cooling.
Buffer Tank Sizing for Cascade Heating
In general, when sizing a parallel cascade heat pump system, defrost energy and peak load shaving play a less significant role compared to a single, larger heat pump. As a result, the required buffer tank volume can be smaller.
The following section provides a more detailed characterization of how parallel and serial (booster) heat pump cascades influence buffer tank sizing. When running system simulations, it’s important to consider that available space is often limited in real-world projects.
Effects of Parallel Cascade Heat Pump Systems on Buffer Tank Design
All heat pumps feed into the heating system or buffer tank at the same temperature level. This has the following implications for buffer tank planning:
- More consistent temperature levels within the tank, allowing for stable thermal stratification.
- More efficient use of the storage volume due to uniform charging.
- Greater flexibility in capacity adjustment through staged operation (e.g., only one heat pump running during partial load).
- Since high temperature differences or intermediate buffer tanks are not required, the overall buffer tank volume can be smaller than in a serial cascade system.
- Easier integration into existing storage structures.
Effects of Series Cascade Heat Pump Systems on Buffer Tank Design
A serial heat pump cascade has the following implications for buffer tank planning:
- Higher supply temperatures are possible compared to a parallel cascade.
- The buffer tank may need to withstand higher temperatures.
- Thermal stratification becomes more complex, as multiple temperature levels can occur within the tank.
- The use of multiple tanks or stratified buffer tanks may be necessary to maintain clear separation of temperature zones.
- A larger storage volume is required to ensure sufficient hydraulic decoupling between the heat pump stages.
How Can a Heat Pump Cascade System Be Precisely Planned and Optimized?
Due to the complex interactions between components and the variable operating conditions, the design of cascade heat pump systems requires dynamic system simulation. Traditional static calculations are not sufficient to accurately represent either the efficiency gains or the hydraulic behavior of such systems.
With specialized simulation software like Polysun, the dynamic performance of heat pump systems can be modeled with precision. Cascade heat pump systems do not operate linearly over the course of a day or year. They instead respond differently to peak loads, partial load conditions, and changing outdoor temperatures. Only a dynamic simulation ensures an accurate planning and optimization.
Simulation of Heat Pump Cascade Systems Is Essential for Accurate Planning
Manual system design cannot fully capture the complexity of cascade configurations. Specialized heat pump software such as Polysun enables hourly—or even minute-by-minute—simulations throughout the entire year. This allows for optimal coordination of all system components.
The simulation is especially important for accurately modeling heat pump cycling behavior, modulation, and efficiency under partial load conditions, as well as stratification dynamics, charging behavior, and thermal losses in the buffer tanks. It also accounts for the influence of mixing valves on return temperatures and storage zones, as well as the hydraulic configuration and overall system control.
This level of detail enables precise control over when each heat pump starts, what temperature levels are maintained, and how thermal energy is distributed throughout the system.
Optimizing Heat Pump Cascades and Avoiding Unnecessary Costs
Only through simulation can the differences in seasonal performance factor (SPF) across various cascade strategies be realistically represented. Operating hours, idle times, and cycling frequency are made transparent. This allows a solid assessment of energy efficiency and operating costs.
Simulation also provides critical advantages in system sizing and investment security. Oversized buffer tanks lead to unnecessary costs, while undersized tanks reduce efficiency and increase heat pump cycling. Simulation helps identify the optimal balance and enables system comparisons—for example, with single heat pumps or hybrid systems. This ultimately benefits building owners, energy providers, and contractors by reducing costs and improving system performance.
Minimizing the Risk of Design Errors
Without simulation using dedicated heat pump software like Polysun, there is a significant risk of system misdesign. Undersized buffer tanks hinder runtime optimization, incorrect hydraulic layouts lead to inefficient partial load operation, and insufficient redundancy compromises operational reliability. Software tools such as Polysun enable a comprehensive evaluation of all system components under realistic conditions. This is something manual calculations simply cannot achieve.
Conclusion:
By simulating the cascade heat pump system and optimizing the cost of the heat pump system, building owners, real estate investors, energy contractors, and energy service providers can reduce energy costs by up to 40%. For both new construction and retrofit projects, simulation provides planning reliability, helps meet CO₂ reduction targets, and minimizes the risk of costly design errors.
FAQ
What Are Typical Mistakes in Planning Cascade Solutions?
When designing and sizing cascade heat pump systems, errors in system configuration can negatively impact efficiency, service life, and operating costs.
Inaccurate Sizing of Cascade Stages
A common issue is incorrect sizing of the cascade stages. If one or more stages are oversized, it leads to frequent cycling—constant switching on and off—which increases wear and significantly reduces efficiency. Particularly problematic is an uneven distribution of output: if one heat pump is much larger than the others, it permanently carries the main load while the rest are barely used. This results in wasted energy and uneven component wear. As a consequence, electricity consumption can increase by 15% to 30%, while the system’s lifespan decreases.
Hydraulic Integration Without Balancing
Another frequent mistake is improper hydraulic integration. Without hydraulic balancing, heat cannot be evenly distributed among the heat pumps, leading to overloading of individual units. Complex hydraulic systems with many mixing circuits or unclear flow paths can cause high pressure losses, requiring more pump power and significantly reducing overall system efficiency.
Control System Deficiencies
Deficiencies in control technology can also have negative effects. Uncoordinated staging—meaning a lack of synchronization regarding which heat pump is activated and in what order—can result in several units starting simultaneously. This creates unnecessary power peaks and stresses the electrical grid.
Insufficient Integration of Domestic Hot Water Production
An often underestimated issue is insufficient integration of domestic hot water production. Buffer tanks that are too small can interrupt cascade operation when hot water is drawn at the same time as heating is required.
These potential planning errors demonstrate that careful and holistic consideration of the cascade heat pump system—both hydraulically and in terms of control—is essential for achieving an efficient, durable, and economical installation.
Are There Cascade Heat Pump Systems With Modulating Heat Pumps?
Yes, there are cascade heat pump systems with modulating heat pumps. They are increasingly preferred, especially in applications with highly variable heat demand, such as multi-family buildings, hotels, or office buildings.
When planning and implementing such cascades, several key aspects must be considered to ensure reliable and efficient operation:
The cascade control system must genuinely support modulation. This means the controller must do more than send simple on/off signals. It must also be able to control the output of each heat pump via suitable interfaces such as 0–10 V, Modbus, CAN, or similar protocols.
Proper hydraulic planning remains essential. Even with modulating units, hydraulic balancing is crucial. In many cases, a buffer tank with a defined minimum volume is required to increase the system’s thermal inertia. This enables stable control.
Without these measures, unstable operation or unnecessary cycling may occur despite modulation capability.
What Is a Multi-Source Heat Pump System?
A multi-source heat pump system, also known as a multi-source or hybrid heat pump system, combines at least two different heat sources to increase the efficiency and reliability of heat supply. Typical sources include ground (via probes or collectors), outdoor air, groundwater, lake water, solar thermal, wastewater, or waste heat. The system is designed to use the most favorable or efficient source at any given time, for example, when a higher source temperature is available or the energy input required for a particular source is especially low.
Advantages of Multi-Source Heat Pump Systems
Higher average source temperature, which significantly increases heat pump efficiency.
Reduced space requirements for ground probes, especially important in urban areas, since part of the heat can be flexibly sourced from alternatives like outdoor air.
High reliability: If one source becomes temporarily unavailable or less efficient (e.g., due to freezing or capacity limits), the system can seamlessly switch to another source.
System Variants
In practice, various system variants are used:
All heat sources may feed a common refrigeration circuit, with one or more evaporators absorbing energy.
Alternatively, several evaporators—one per source—may be connected in series or parallel within a single refrigeration circuit.
Some systems have multiple heat pumps for different sources connected to a shared heating system.
For example, a large multi-family building can be supplied by a multi-source system using ground and outdoor air. In planning, multi-source heat pump systems are necessary to overcome typical barriers to sourcing heat—such as space constraints or noise emissions—and to reduce investment costs.
Why Simulate a Multi-Source Heat Pump System?
Especially for multi-source heat pump systems, simulation of the heat pump system is essential. Only simulation can realistically map the complex interaction of different heat sources, dynamic operating conditions, and site-specific factors. Using simulation, different system variants and operating strategies can be analyzed in detail in advance. This helps avoid planning errors, maximize efficiency, and improve economic viability. Potential issues, such as insufficient heating output or excessive electricity consumption, can be identified and resolved early. This prevents costly corrections and operational failures.
How Do You Precisely Size a Cascade Heat Pump System With Simulation Software?
Input of System Parameters
Precise sizing of a cascade heat pump system with simulation software occurs in several steps that model real operating behavior and all relevant system parameters. First, building data, heating and domestic hot water demand, and site conditions are entered into the software. Using dynamic weather and load profiles, the software simulates actual heat demand throughout the year. Market-standard heat pumps can then be selected from a component database and configured as individual cascade stages in the system model. Hydraulic connections—including buffer tanks, pumps, and controls—are set up according to the planned system.
Simulation of Real Operating Behavior
Next, the software simulates the operation of the entire cascade over a typical year. For each heat pump and each time step, source temperatures, supply temperatures, electrical input, and heat output are calculated. Cascade control and different switching strategies can be realistically modeled and optimized, allowing the optimal number and capacity of individual heat pump stages to be determined. The goal is even utilization, minimal cycling, and high efficiency.
Variant Comparison and Optimization
A key advantage of simulation software is the ability to compare and optimize different design variants: for example, different cascade configurations, storage sizes, or control strategies can be directly compared. The software provides detailed evaluations of electricity consumption, seasonal performance factor, runtimes, and temperature profiles for each cascade stage and the overall system. This allows variants to be optimized for project goals such as CO₂ savings or economic efficiency. The results report can transparently present CO₂ savings and annual operating costs, considering energy consumption, component lifespan, and rising energy prices.
This methodological approach ensures the cascade heat pump system is precisely tailored to project requirements and operates optimally under real conditions.
For Which Building Types Are Heat Pump Cascade Solutions Suitable?
Cascade heat pump systems are particularly suitable for buildings and applications with high or highly variable heating, domestic hot water, or cooling demand. Typical use cases include:
Multi-family buildings and larger residential complexes: Ideal due to high and often variable heating and hot water demand.
Office and administrative buildings: Heating and cooling demand fluctuates significantly by use and time of day. Cascades enable flexible output adjustment and efficient part-load operation.
Schools and educational institutions: Schools have varying heating and hot water demand (e.g., mornings, midday, sports activities). Cascades provide the necessary flexibility and redundancy to ensure operation even if individual units fail.
Hotels, sports facilities, care homes: In buildings with high and simultaneous demand for space heating and domestic hot water (e.g., hotels, fitness centers, care and senior centers), cascades are especially advantageous, enabling simultaneous provision of heating, hot water, and possibly cooling.
Commercial and industrial properties:
Cascades offer scalability and reliability in commercial buildings, production facilities, or warehouses with large or fluctuating heat demand.
Older buildings with variable demand: In older buildings with different insulation standards and variable heating loads, cascades can optimally adjust output and improve efficiency.
Large single-family homes with special requirements: Even in large single-family homes, such as those with a separate apartment or home office, a cascade may be worthwhile when high flexibility or redundancy is needed.
How Many Units Are Optimal in a Cascade for a Large School Building?
he optimal number of units depends on several factors. For a larger school building, the ideal number of heat pumps in a cascade is typically between two and three. Practical examples show that for high heating and domestic hot water demands—about 150 kW or more—two powerful heat pumps are often used in cascade.
For large schools with very high energy needs, the cascade may consist of more than three units in rare cases, but this should be verified with a detailed simulation. In most cases, two to three heat pumps are optimal to combine efficiency, cost-effectiveness, and operational reliability.
How Does the Seasonal Performance Factor (SPF) Change With a Heat Pump Cascade?
Compared to a single-unit system, a cascade heat pump system can increase the seasonal performance factor (SPF), since the individual units operate more frequently in their optimal operating range and can be better matched to demand. This requires efficient control and system design. However, the exact SPF must always be determined individually for each system and building, as it is influenced by numerous factors.
How Do I Integrate a Heat Pump Cascade Into Existing Heating Systems (Hybrid Solutions)?
Integration of a cascade heat pump system into existing heating systems usually occurs as part of a hybrid solution, where the cascade is combined with another heat generator (e.g., gas or oil condensing boiler, solar thermal, or photovoltaics). The goal is to maximize the benefits of both systems and ensure high efficiency and supply security.
Steps for Integration
Assessment and Planning:
First, the existing heating system is analyzed to determine the best way to integrate the cascade. Key considerations include heat demand, existing infrastructure, and the desired operating strategy (monovalent, bivalent, parallel, or alternative).
Cascade Control and Regulation:
The cascade is equipped with intelligent control to coordinate both the operation of the individual heat pumps and their interaction with the existing heat generator.
Defining the Bivalence Point:
In hybrid solutions, the cascade heat pump system covers the base load, while the conventional heat generator (e.g., gas or oil boiler) is activated during very low outdoor temperatures or peak loads.
Hydraulic Integration:
The cascade is integrated into the existing heating network, usually via a buffer tank that acts as a hydraulic separator and ensures flexible supply to all consumers. Existing heating systems and pipe diameters must be considered.